This invention relates to a printing blanket, and more particularly to a printing blanket having an improved dynamic thickness stability and resistance to permanent gauge loss.
In offset lithography, a rotary cylinder is covered with a printing plate which normally has a positive image area receptive to oil-based inks and repellent to water and a background area where the opposite is true. The printing plate is rotated so that its surface contacts a second cylinder covered with a rubber-surfaced ink-receptive printing blanket (sometimes also called a printer's blanket). The ink present on the image surface of the printing plate transfers, or offsets, to the surface of the blanket. Paper or other sheet stock to be printed is then passed between the blanket-covered cylinder and a rigid back-up cylinder to transfer the image from the surface of the blanket to the paper.
During the step in which the inked image is transferred from the plate to the blanket and the step where the image is transferred from the printing blanket to the paper, it is important to have intimate contact between the two contacting surfaces. An exact amount of interference pressure is required so that the blanket contacts and removes ink from the image plate and transfers the inked image to a proper depth into the paper. This is ordinarily achieved by positioning the blanket-covered cylinder and the supporting cylinder it contacts so that there is a fixed interference between the two and so that the blanket is compressed throughout the run to a fixed depth, typically approximately 0.05 to 0.10 mm (0.002 to 0.004 inches). It is important that this compression be maintained uniformly over the entire surface of the blanket.
Within the current state of the art, all printing blankets lose thickness (i.e., lose gauge or "sink") when they are initially tensioned and installed, and further lose thickness as the blanket is repeatedly exposed to the interference pressures at the nips between the printing cylinder and blanket-covered cylinder and the blanket-covered cylinder and rigid back-up cylinder, respectively. Blankets can fail catastrophically due to blanket smash, a permanent deformation in a portion of the entire blanket surface, or from a gradual deterioration of blanket gauge over time due to the repeated cycling of the interference pressures on the blanket's surface. When the thickness of a blanket recedes beyond the limits of press adjustment, the print pressure becomes insufficient to cause transfer of the inked image from the print cylinder to the blanket or the blanket to the paper, or both. Thus, for a typical blanket, a permanent loss of thickness of as little as 0.05 to 0.10 mm (0.002 to 0.004 inches) may require a press stoppage.
Conventionally, the fixed interference described above is accomplished by inserting one or more thin layers of paper or the like between the blanket and the surface of the blanket cylinder to build up the thickness of the blanket. This process is known as packing a blanket. Once the gauge loss of the blanket reaches a certain amount, as described above, additional thickness must be supplied under the blanket. This involves stopping the press, demounting the blanket and original packing, repacking, and then remounting and retensioning the blanket.
The packing process presents problems however in that the procedure is time consuming, resulting in down time for the printing equipment. Typically, press downtime can cost from several hundred to over a thousand dollars per hour. It may take over 30 minutes to pack or repack a blanket. Further time is lost as the system is retuned to optimum settings. Additionally, once positioned on the cylinder, the packing paper tends to slide, slip, and/or fold which may render the blanket surface nonuniform and result in poor printing results.
To avoid some of the problems associated with packed blankets, some press operators, and in particular news press operators, have used blankets which do not require packing. So-called "no pack" blankets have been developed to provide a fixed interference without the need to pack the blanket. No pack blankets are manufactured to very precise gauges so that they can be installed directly onto a blanket cylinder with the correct amount of interference. These blankets have the advantage of a one-piece construction which requires no positioning of packing paper beneath the blanket. This results in less down time for the printing equipment when an old blanket is removed and replaced with a new blanket.
Such no pack blankets, like most printing blankets, are normally composed of a base material which gives the blanket dimensional stability. Woven fabrics are preferred. The base may consist of one or more layers of such fabric. The working surface of the blanket which contacts the ink is typically an elastomeric layer of natural or synthetic rubber which is applied over the base layer or layers. The base layer or layers and working surface are laminated together using suitable adhesives. Again, such blankets exhibit some gauge loss upon initial tensioning and installation and continue to lose thickness over time during use. However, once the gauge (thickness) loss on a no pack blanket exceeds the limits of press adjustment, the blanket becomes unusable without recourse and must be replaced by a new blanket.
Accordingly, there remains a need in the art for a printing blanket which resists gauge loss throughout its useful life. Such a blanket would reduce expensive down time for press operators and require fewer adjustments of the press during operation.